This invention relates generally to xerographic process control, and more particularly, to a programmable technique in which a prestored value or count is used to control the extent to which an imaging member is exposed by a beam for the purpose of generating a developer material patch.
In copying or printing systems, such as a xerographic copier, laser printer, or ink-jet printer, a common technique for monitoring the quality of prints is to artificially create a "test patch" of a predetermined desired density. The actual density of the printing material (toner or ink) in the test patch can then be optically measured to determine the effectiveness of the printing process in placing this printing material on the print sheet.
In the case of xerographic devices, such as a laser printer, the surface that is typically of most interest in determining the density of printing material thereon is the charge-retentive surface or photoreceptor, on which the electrostatic latent image is formed and subsequently, developed by causing toner particles to adhere to areas thereof that are charged in a particular way. In such a case, the optical device for determining the density of toner on the test patch, which is often referred to as a "densitometer", is disposed along the path of the photoreceptor, directly downstream of the development of the development unit. There is typically a routine within the operating system of the printer to periodically create test patches of a desired density at pre-established locations on the photoreceptor by deliberately causing the exposure system thereof to charge or discharge as necessary the surface at the location to a predetermined extent.
It is also understood that the amount of charge applied to the photoreceptor can be controlled through monitoring of a voltage in the exposed test patch area. Commonly, this is accomplished through employment of an electrostatic voltmeter ("ESV") disposed adjacent the photoreceptor, upstream of the development unit.
The latent test patch pattern is then moved past the developer unit and the toner particles within the developer unit are selectively electrostatically attracted to the test patch pattern. The quantity or density of the toner on the test patch causes proportional darkness or saturation of the test patch in optical measurement. The developed test patch is thus moved past a densitometer disposed along the path of the photoreceptor, and the light absorption of the test patch is tested. The proportion of light that is absorbed by the test patch correlates closely to the quantity of toner on the test patch. As a result, the printing system can use the densitometer signal to ascertain the correct level of development or proper amount of toner or ink deposition.
In any printing system using test patches for monitoring print quality, a design problem inevitably arises of where to place these test patches, particularly on photoreceptor belts or drums. Xerographic test patches are traditionally printed in the interdocument zones between printed images of a photoreceptor having multiple "pitches". These patches are used to control the deposition of toner on images printed on paper in order to monitor and regulate the tone reproduction curve (TRC) of the printer. Generally each patch area may be about an inch square, and is selectively printed as a uniform area that is one of: solid half tone, solid (saturated), or halftone (patterned gray or background (white)).
This practice enables the sensor to read one point value on the tone reproduction curve for each test patch corresponding to its density. However, that is insufficient to complete the measurement of the entire curve at reasonable intervals, especially in a multi-color print engine. To have an adequate number of points on the curve, multiple test patches have to be created. Thus, the traditional method of process controls involves scheduling solid area, uniform halftones or background in a test patch. Some of the high quality printers contain many test patches. During the print run, each test patch is scheduled to have single halftone that would represent a single point byte (8-bit binary) value on the tone reproduction curve (TRC). Full scale TRC's are commonly defined in 8-bit weighted binary values, representing 256 levels of saturation from "white" to "solid saturated".
Various prior art techniques have been proposed to improve the use of test patches for xerographic control. For example, U.S. Pat. No. 5,543,896 to Mestha discloses a method of development control by storing a reference tone reproduction curve and providing a single test pattern including a scale of pixel values in the interdocument zone of the imagining surface. The system senses the test pattern along the scale of pixel values in the interdocument zone and responds to the sensing of the test pattern and the reference tone reproduction curve to adjust the machine operation for print quality correction. It is also known in the prior art to image multiple test targets in the interdocument zones of the photoreceptor (see e.g. U.S. Pat. No. 4,341,461 to). For example, two test targets each having two test patches are selectively exposed singly or in overlapping relationship to provide test data to control toner dispensing and developer bias.
Despite process control advancements of the type discussed above, there are still difficulties associated with placing test patches accurately on photoreceptive surfaces. One solution for placing a test patch across a photoreceptor, i.e. placing the test patch in a direction perpendicular to the process direction of the photoreceptor otherwise described as the "fast scan" direction of the photoreceptor, is disclosed by U.S. Pat. No. 4,949,105 to Prowak. In the '105 Patent a pixel counter is used in conjunction with two comparators for delineating the extent to which a test patch is written on the photoreceptor in the fast scan direction. More particularly, patch writing control is achieved with the logical circuit of FIG. 5 in such a manner that the patch is written, in accordance with data stored in one of registers 84 and 86, provided the Patch Print Enable signal is enabled and the patch address (in the fast scan direction) is between the Patch Start Address and Patch Stop Address. Control of patch writing in the process direction of the photoreceptor, i.e. the "slow scan" direction of the photoreceptor, is achieved through suitable timing. U.S. Pat. No. 4,949,105 does not suggest that the control of writing in the slow scan direction is critical and it is believed the '105 Patent test patch could be significantly longer in the slow scan direction than disclosed without affecting the concept upon which the invention of the '105 Patent is based.
In certain conventional printers, particularly high speed printing machines in which test patches are written in the interdocument zone (see e.g. U.S. Pat. No. 5,652,946 to Scheuer et al.), control of patch writing in the slow scan direction is of significant concern. It is known that one conventional copier writes a test patch in the slow scan direction of the interdocument zone by "syncing" a writing signal off of a signal from a marking imaging subsystem. That is, the subsystem provides a signal that controls how long patch exposure or discharge should be enabled in the interdocument zone. Essentially, the writing signal is customized for the host machine and when the speed of the photoreceptor of the host machine is increased, redesign of the mark imaging subsystem is typically required. This sort of redesign can be relatively expensive and time consuming.
Accordingly, it would be desirable to provide a relatively inexpensive, flexible patch writing system that need not be redesigned upon altering the speed of the photoreceptor with which the patch writing system is associated.